1. Field of the Invention
The present invention relates to an image-forming apparatus in which, after a charged-developing agent is affixed to the surface of a recording medium in image-wise arrangement, the image in the developing agent is fixed on the recording medium, thereby forming the image on the recording medium.
2. Description of the Related Art
An example of this type of image-forming apparatus is a widely known electrophotographic image-forming apparatus. As described in, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-328552, this electrophotographic image-forming apparatus includes a photoconductor drum, a charging device, an exposing device, a developing device, a transfer roller, a fixing device, and a transport guide.
The photoconductor drum is disposed in a process cartridge, which partially constitutes image-forming means. The charging device is configured so as to uniformly charge the surface of the photoconductor drum. The exposing device is configured so as to expose the surface of the photoconductor drum which is uniformly charged by the charging device, thereby forming an electrostatic latent image on the surface.
The developing device is configured so as to affix charged toner (developing agent) to the surface of the photoconductor drum on which the latent image is formed, in an image-wise pattern corresponding to the latent image. In other words, this developing device is configured so as to develop the latent image by use of the toner. Hereinafter, an image which is formed through attachment of the developing agent (toner) in image-wise arrangement is simply called a “developing-agent image (toner image).”
The transfer roller is configured so as to electrostatically transfer the toner image from the surface of the photoconductor drum to the surface of the recording medium. The fixing device is configured so as to fix the toner image on the surface of the recording medium through application of heat and/or pressure to the recording medium on which the toner image has been transferred by means of the transfer roller. The transport guide is configured so as to guide transport of the recording medium which has passed the transfer roller, toward the fixing device.
In the image-forming apparatus described in Japanese Patent Application Laid-Open (kokai) No. 2002-328552, the transport guide includes transport ribs and a metal sheet. A static-eliminating member is provided upstream of the transport guide with respect to the recording-medium transport direction.
The metal sheet is grounded. This metal sheet is configured so as to stably transport the recording medium through electrostatic attraction of the non-image side (back surface) of the recording medium which has passed the transfer roller.
The transport ribs are formed from a synthetic resin and provided in such a manner as to project toward the recording medium. The transport ribs are configured so as to lower transport resistance of the recording medium and to restrain occurrence of an abnormal image which could otherwise result from disturbance of the unfixed toner image during passage of the recording medium through the transport guide.
Conceivably, occurrence of an abnormal image is due to an exchange of charges between the back surface of the recording medium and the transport guide at the time the recording medium bearing the unfixed toner image comes in proximity to or in direct contact with the electrically conductive metal sheet.
The static-eliminating member is formed of a grounded, electrically conductive member and is configured so as to eliminate static charges from the back surface of the recording medium. That is, through elimination of static charges from the back surface of the recording medium by means of the static-eliminating member, the recording medium which has passed the transfer roller can be smoothly transported toward the transport guide and the fixing device.
The thus-configured image-forming apparatus encounters extreme difficulty in completely solving the above-mentioned problem of occurrence of an abnormal image during passage of the recording medium through the transport guide.
For example, if the amount of static charges on the back surface of the recording medium is excessively large or small, the above-mentioned abnormal image arises remarkably. This abnormal image arises in the form of streaky “trace of ribs” along the transport direction at positions corresponding to the transport ribs or radial “scatter”.
Conceivably, “trace of ribs” arises through the following mechanism. In the case of accumulation of an excessively large amount of static charges on the back surface of the recording medium, the electrostatic attraction between the recording medium and the metal sheet becomes excessively strong. This excessively strong, electrostatic attraction causes strong friction between ends of the transport ribs and the back surface of the recording medium moving in the transport direction. This strong friction generates charges. The thus-generated charges cause adsorption or scatter of the toner; i.e., charged particles. Also, discharge of the charges causes scatter of the toner. As a result, a streaky disturbance of image arises along the transport direction.
Conceivably, “scatter” arises through the following mechanism. In the case of accumulation of an excessively large amount of static charges on the back surface of the recording medium, discharge occurs between the back surface and the transport guide. This discharge causes scatter of the toner; i.e., charged particles. As a result, a radial disturbance of image arises along the transport direction.
Meanwhile, various adverse effects also arise when the amount of static charges on the back surface of the recording medium becomes excessively small as a result of excessive elimination of static charges from the back surface of the recording medium by means of the static-eliminating member, which is located upstream of the transport guide with respect to the transport direction. In this case, the electrostatic attraction between the recording medium and the metal sheet becomes excessively weak, thereby causing loss of stability of transport of the recording medium. In this case, the above-mentioned “scatter” also arises.
Conceivably, “scatter” in this case arises through the following mechanism. Excessive elimination of static charges from the back surface of the recording medium causes a relative drop in an electrostatic adsorbing force with which the toner adsorbs on the recording medium. Then, for example, during transport of the recording medium, a certain (mechanical or electrostatic) impact exerted locally on the recording medium causes scatter of the toner. As a result, a radial disturbance of image arises along the transport direction.
The above-mentioned abnormal image which occurs during passage of the recording medium through the transport guide is particularly likely to occur on the second side of a sheet when both sides of a sheet undergo image-forming. Conceivably, this is because a drop in water content of the recording medium and the occurrence of a curl of the recording medium as a result of passage through the fixing device cause a change in the degree of friction of the recording medium against the transport ribs and in electrostatic propensity of the recording medium.
However, controlling the electrostatically charged condition of the back surface of the recording medium by means of the static-eliminating member is very difficult. This is because the condition of electrostatic charge varies greatly depending on, for example, the environment in which the image-forming apparatus is used, and the material of the recording medium. Also, as mentioned above, the condition of the electrostatic charge could differ greatly between the first side and the second side in double-sided image formation.
In this connection, appropriately controlling the electrostatic attraction between the recording medium and the metal sheet through application of a predetermined bias voltage between the metal sheet and the ground potential is also difficult. This is because the polarity of and the amount of static charges on the back surface of the recording medium could vary depending on the material of the recording medium and the materials of elements disposed in the image-forming apparatus, and thus appropriately setting the polarity of the bias voltage is difficult.
Particularly, when the material of the recording medium and the materials of elements disposed in the image-forming apparatus are positioned close to one another on a frictional electrification sequence, the polarity of static charges on the back surface of recording medium cannot be fixed.
Furthermore, the configuration of the image-forming apparatus becomes complex; for example, necessity arises for a power unit to be connected to the metal sheet, and a circuit for this connection. As a result, provision of an inexpensive image-forming apparatus becomes difficult.